This invention relates generally to determining position by electromagnetic radiation. More particularly, the invention relates to an improved system for transmitting position data between vehicles.
As the world becomes a more crowded and busy place, vehicular traffic, on land, on sea and in the air has correspondingly become more crowded. This has resulted in the increased likelihood of collisions with other vehicles as well as with stationary objects. In addition, a need has been recognized to manage the increased traffic levels in a more efficient manner to allow the greatest use of the highways and airways. Anticollision devices have been developed which alert the operators of the vehicles in the event of an imminent collision so that evasive actions may be undertaken. So called smart roads have been proposed to allow properly configured land based vehicles such as automobiles and trucks to be controlled by a central agency.
The problem of collisions is particularly acute with aircraft as the potential for a catastrophic loss of life is great. The Federal Aircraft Administration (FAA) has mandated that all aircraft having more than 30 seats be equipped with collision avoidance equipment generally known as Traffic Alert and Collision Avoidance Systems (TCAS). The commercial TCAS system relies on the continuous operation of Air Traffic Control Radar Beacon System (ATCRBS) transponders in all private, commercial and military aircraft. There are a variety of types of ATCRBS transponders in use which have a number of deficiencies. For instance, not all of the transponders transmit altitude information making collision avoidance somewhat problematic. Another problem is that the TCAS are limited in areas of high traffic volume where system saturation causes transponder signals from different aircraft to overlap. For example, if two aircraft are equidistant from the radar transmitter, they will respond at the same time and overlay each other's signal. Furthermore, the timing of the return signal must be accurately measured to help determine the location of the aircraft. A greater problem, however, is the high cost of the TCAS which has inhibited its widespread use in noncommercial aircraft.
A number of references have suggested that the Global Positioning System (GPS) made be used as part of a collision avoidance system. GPS is currently the most precise positioning system generally available to the general public and has significantly dropped in price in recent years. The GPS comprises a network of 24 satellites orbiting the earth. Each satellite transmits a ranging signal modulated on a 1.575 Ghz carrier. By monitoring the signal from a plurality of satellites, a GPS receiver can determine its position, i.e. latitude, longitude and altitude, to an accuracy of about 100 meters. In general, an aircraft would need to receive signals from four of the GPS satellites for an altitude measurement. More accurate signals are available to the military. Differential GPS, also available to the public, is more accurate than standard GPS, but requires an additional land based transmitter and special permission from the government.
Although many of the proposed GPS-based systems are simpler and cheaper than the TCAS presently in use which relies on transponders and interrogations, in areas of high traffic volume the number of transmissions and receptions which must occur can cause the systems to saturate. One notable effort to alleviate the problems of a GPS-based system is taught by "Traffic Alert and Collision Avoidance Coding System", U.S. Pat. No. 5,636,123 to Rich et al. In the Rich system, the airspace is divided up into a grid of volume elements. A collision avoidance signal is transmitted wherein the carrier signal is modulated by a psuedonoise code which is function of the volume element in which the aircraft is located. Each aircraft only tracks collision avoidance signals from vehicles in its own and immediate surrounding cells. Based on the calculated paths of the aircraft, a warning of an impending collision can be provided to the pilot.
In the opinion of the Applicants, the Rich system suffers from a number of deficiencies. First, the signal they transmit is modulated by a pseudo random signature signal, called a "gold code", which needs to be received with expensive reconfigurable correlation receivers. The Rich systems transmit and depend on carefully chosen "gold codes" which are intended to produce distinct correlation peaks. The receiver has to be capable of searching for the signals. The requirement of complicated transmission and reception equipment frustrates the objective of having a system which is inexpensive enough to be generally available to the public. While there is perhaps less price sensitivity for aircraft, it has been demonstrated that the high cost of the TCAS has prevented its widespread adoption in noncommercial aircraft. Furthermore, as the reader will understand, the Applicants believe that the present invention has a more general applicability to land and water vehicles which have higher price sensitivity.
The volume elements proposed by Rich are rather coarse, on the order of 5 miles on a side. Despite a psuedorandom multiplexing feature when multiple transmitters transmit in the same cell, noise and other saturation effects can occur when there is too much traffic in the rather large volume element. The large volume element allows the Rich system to only look at the aircraft's element and those immediately adjacent. For some applications, it would be preferable to understand the vehicles present in an extended set of cells surrounding the vehicle. For example, it might be extremely difficult to use the Rich system as a central control facility. An airport control center, for example, may have a very difficult time reading signals from more distant volume elements with the vehicles in closer cells all transmitting simultaneously.
Although the Rich system uses the accurate GPS time to set the phase of their encoding signals, these can still be subject to delays from the speed of light propagation causing phase shifts. Their coding scheme causes "interference annuli" which prevent the craft from hearing each other at certain times. The system uses the GPS time as part of the input to the transmission process, but does not use it to prevent simultaneous transmissions.
This invention solves these and other important problems.